1. Field of the Invention
The present invention relates to a detection probe respectively used in a method and a device for measuring an AC voltage applied to a conductor insulated by insulation, such as a vinyl insulated electrical wire, without contacting the conductor.
2. Description of the Prior Art
Conventionally, an AC voltmeter is generally used in the measurement of a voltage of a commercial AC current (supply) being applied to an insulated electrical wire.
However, with a method using a conventional AC voltmeter, since it is necessary to bring one measurement electrode into contact with a conductor, it was necessary to cut away part of the insulation of the insulated electrical wire, or to provide a terminal for measurement in advance.
The applicant of the present application has previously proposed a non-contact voltage measurement method and a device that operates at low voltage and is portable (Japanese Patent No. 3158063).
FIG. 13 is a block diagram showing the structure of a non-contact voltage measurement device 80 of the related art, and FIG. 14 is an equivalent circuit of essential parts of the voltage measurement device 80.
In FIG. 13, the voltage measurement device 80 comprises a detection resistor R1, a detection probe 11, an oscillator 12, a current detection section 13j, a band pass filter 21, a rectifier 22, a capacitance calculation section 23, a floating capacitance detection section 24, a switch 25, a low pass filter 26, an integrator 27 and a divider 28.
The detection probe 11 is provided with a detection electrode 111 for electrostatically shielding part of a conductor CD, by covering part of the surface of insulation SL of an electrical wire WR from the outside, and a shield electrode 112 for electrostatically shielding the detection electrode 111 from the outside. Impedance Zw between the detection electrode 111 and the conductor CD is measured using the detection probe 11. In practice, reactance XC1, namely capacitance C1, is measured in place of the impedance Zw.
Composite capacitance due to capacitance between the probe electrode 111 and the shield electrode 112, floating capacitance due to wiring to the detection resistor R1, and other floating capacitance, is made C0, and reactance due to this is made XC0. Capacitance C0 is sometimes referred to as xe2x80x9cfloating capacitance C0xe2x80x9d.
Electric current flowing from the oscillator 12 through the detection resistor R1 into the detection electrode 111 is Is, and electric current discharged from the detection electrode 111 towards the oscillator 12 is Ix. Within current Is, there is current Is0 flowing through the floating capacitance C0 to a ground terminal, and a current Is1 flowing through the capacitance C1 and the conductor CD to the ground terminal.
The oscillator 12 outputs, for example, a 5 KHz sine wave signal of a certain voltage Es. The current detection section 13j detects current flowing into the detection electrode 111 and current discharged from the detection electrode 111, and outputs a signal S1.
Within the signal S1 output from the current detection section 13j, the band pass filter 21 allows only a component due to the signal Es of the oscillator 12 to pass. The floating capacitance detection section 24 measures and stores floating capacitance C0 with the detection probe 11 separated from the electrical wire WR. The capacitance calculation section 23 calculates capacitance C1 based on a signal S3 output from the rectifier 22 with the surface of the insulation SL covered by the detection probe 11, and the floating capacitance C0 stored in the floating capacitance detection section 24. The obtained capacitance C1 is output to the divider 28 as a signal S4.
Within the signal S1 output from the current detection section 13j, the low pass filter 26 allows only a component due to the voltage Ex applied to the conductor CD to pass. The integrator 27 integrates a signal S5 output from the low pass filter 26. In this way, phase compensation of the signal waveform is carried out. The divider 28 obtains a voltage Ex by dividing the signal S6 (Erx) output from the integrator 27 by the signal S4 (capacitance C1) output from the capacitance calculation section 23.
First of all, the floating capacitance C0 is measured with the detection probe 11 in an open state. The impedance Z0 seen from the oscillator 12 side is:
Z0=R1+j XCx0
but since the detection resistor R1 can be ignored compared to the reactance XC0, the impedance Z0 becomes:
Z0=XC0
Accordingly, current Is0 flowing into the floating capacitance C0 due to the signal Es output from the oscillator 12 is:                                                                         Is                0                            =                              Es                /                Z0                                                                                        =                              Es                /                XC0                                                                        (        1        )            
while a voltage Er across the two ends of the detection resistor R1 due to this current is:   "AutoLeftMatch"                                                                        Er                =                                                      Is                    0                                    xc3x97                  R1                                                                                                        =                                                      (                                          Es                      xc3x97                      R1                                        )                                    /                  XC0                                                                                          (          2          )                    
and therefore:
XC0=(Esxc3x97R1)/Er C0=(Esxc3x97R1)/xcfx89sxc2x7Erxe2x80x83xe2x80x83(3) 
A value of floating capacitance Co obtained from this equation (3) is stored in the floating capacitance detection section 24. Next, capacitance C1 is measured with the detection probe 11 closed.
The capacitance C1 is increased by the fact that the detection electrode 111 covers the electrical wire WR. Accordingly, impedance Zw seen from the oscillator 12 is:
Zw=R1+j XCcxe2x80x83xe2x80x83(4) 
provided that, Cc=C0+C1
Since the detection resistor R1 is small compared to reactance XCc, and can be ignored,
Zw=XCc 
Accordingly, current Is flowing into the detection resistor R1 due to the signal Es output from the oscillator 12 is:                                                         Is              =                              Es                /                Zw                                                                                        =                              Es                /                XCc                                                                        (        5        )            
while a voltage Er developed across the two ends of the detection resistor R1 by this current is:                                                         Er              =                              Is                xc3x97                R1                                                                                        =                                                (                                      Es                    xc3x97                    R1                                    )                                /                XCc                                                                        (        6        )            
Accordingly, since:
XCc=(Esxc3x97R1)/Er 
and XCc is xcfx89s (C0+C1),
C0+C1=(Esxc3x97R1)/xcfx89s xc2x7Erxe2x80x83xe2x80x83(7) 
A value of capacitance (C0+C1) obtained from this equation (7) is input to the capacitance calculation section 23 as a signal S3. In the capacitance calculation section 23, capacitance C1 is obtained by subtracting the value of floating capacitance C0 stored in the floating capacitance detection section 24 from the input value of capacitance (C0+C1), and this is output to the divider 28.
Next, a voltage Er developed across the two ends of the detection resistor R1 attributable to the voltage Ex applied to the conductor CD is obtained with the detection probe 11 closed.
Impedance Zx of the circuit through the detection resistor R1 seen from the conductor CD side is:
Zx=R1+j XC1xe2x80x83xe2x80x83(8) 
Since the detection resistor R1 is small compared to reactance XC1, and can be ignored,
Zx=XC1
Accordingly, current Ix flowing into the detection resistor R1 due to the voltage Ex applied to the conductor CD is:                                                         Ix              =                              Ex                /                Zx                                                                                        =                              Ex                /                XC1                                                                        (        9        )            
A voltage Er developed across the two ends of the detection resistor R1 by this current is:                                                         Er              =                              Ix                xc3x97                R1                                                                                        =                                                (                                      E                    xc3x97                    XR1                                    )                                /                XC1                                                                                        =                              ω                xc3x97                XC1                xc3x97                                  (                                      Ex                    xc3x97                    R1                                    )                                                                                        (        10        )            
A value of voltage Er (Ers) obtained from this equation (10) is input to the divider 28 as a signal S6. The voltage Ex is obtained by the divider 28, by dividing the input voltage Er by a coefficient containing the capacitance C1 output from the capacitance calculation section 23. That is,
Ex=Er/(xcfx89xxc3x97C1xc3x97R1)xe2x80x83xe2x80x83(11) 
is obtained.
However, if the above described voltage measurement device 80 of the related art is used, there is influence from floating capacitance between the detection electrode 111 and the shield electrode 112, and from floating capacitance due to the wiring. For this reason, it is necessary to measure the floating capacitance C0 with the detection probe 11 separated from the electrical wire WR, and time is required for measurement.
Also, the floating capacitance C0 is not fixed and varies depending on the diameter of the conductor CD, conditions of fitting conductor CD to the detection probe 11, conditions surrounding the detection probe 11 etc. Measurement errors arise because of variations in the floating capacitance C0. Overall measurement accuracy is also affected by floating capacitance C0 measurement errors.
Only capacitance C1 was measured as impedance Zw of the insulation SL of the electrical wire WR, but in practice this measurement is affected by leakage resistance of the insulation SL (insulation resistance). Particularly, if the temperature of the electrical wire WR rises, insulation resistance of the insulation SL is lowered, and the effect of this can not be ignored, and this causes measurement errors.
An object of the present invention is to provide a non-contact measurement method and device, and a detection probe used with this method and device, that is not affected by floating capacitance, and that can measure a voltage in a non-contact manner through a simple operation and without measuring floating capacitance.
According to a first aspect of the present invention, there is provided a method for measuring an AC voltage applied to a conductor, without contacting the conductor, using a detection probe provided with a detection electrode, capable of covering part of a surface of insulation for insulating the conductor, and a shield electrode for covering the detection electrode, and an oscillator for outputting a signal having a certain frequency, wherein one end of each of a core wire and a sheath wire of a shield cable are connected to the detection electrode and the shield electrode, and the effect of floating capacitance is substantially made zero by establishing an imaginary short condition between each of the other ends, comprising the steps of:measuring impedance between the detection electrode and the conductor by applying the signal from the oscillator to the detection electrode via the shield cable;measuring current discharged from the detection electrode attributable to a voltage applied to the conductor; and obtaining the voltage applied to the conductor based on the measured impedance and the measured current.
According to another aspect of the present invention, with respect to the other end of the shield cable, an imaginary short-circuit is established by connecting the core wire and the sheath wire to an inverting input terminal and a non-inverting input terminal of an operational amplifier, and a voltage applied to the conductor is obtained by applying the signal from the oscillator to the detection electrode via the shield cable to measure impedance between the detection electrode and the conductor.
A device of one aspect of the present invention comprises a detection probe, provided with a detection electrode capable of electrostatically shielding part of the conductor by covering part of the surface of the insulation from the outside, and a shield electrode for electrostaticaly shielding the detection electrode from the outside, a shield cable, having one end of a core wire and a sheath wire connected to the detection electrode and the shield electrode, an operational amplifier for, with respect to other ends of the shield cable, establishing an imaginary short circuit condition by connecting the core wire and the sheath wire to an inverting input terminal and a non-inverting input terminal, an oscillator for outputting a signal having a certain frequency, and a measurement section, for applying the signal from the oscillator to the detection electrode via the shield cable to measure impedance between the detection electrode and the conductor, and obtaining the voltage applied to the conductor based on the measured results.
A device of another aspect of the present invention comprises an oscillator for applying a signal having a certain frequency to the detection electrode via the shield cable, a detection resistor, for detecting current discharged from the detection electrode attributable to the signal, and detecting current discharged from the detection electrode attributable to a voltage applied to the conductor, and a measurement section for obtaining the voltage applied to the conductor based on a voltage developed across the two ends of the detection resistor.
There is preferably a correction circuit for correcting an error of the operational amplifier so that the imaginary short-circuit state is made close to perfect.
There is also an inverting amplifier having the signal input to an input terminal, and one end of the detection resistor connected to an output terminal, and forming a feedback circuit by way of an inverting input terminal and a non-inverting input terminal of the operational amplifier.
There is also provided a current detection transformer, provided stretching across a housing and a cover so as to be capable of opening and closing together with opening and closing of the cover.
A detection probe of one aspect of the present invention comprises a housing, a cover capable of being opened and closed with respect to the housing, a detection electrode, provided stretching across the housing and the cover so as to be capable of opening and closing together with opening and closing of the cover, having a movable section that can be pressed by applying force towards the insulation, and being capable of covering a surface of part of the insulation, and a shield electrode for covering the detection electrode, provided stretching across the housing and the cover so as to be capable of opening and closing together with opening and closing of the cover.
In the present invention, in order to measure a voltage applied to a conductor, impedance between the detection electrode and the conductor, and a current discharged from the detection electrode attributable to the voltage applied to the conductor are measured, but this is the principal for obtaining a voltage, and it is not at all necessary to measure those physical amounts. It is possible to measure various other physical amounts or parameters to finally obtain a voltage applied to a conductor.
Also, when measuring impedance between the detection electrode and the conductor, resistance and inductance can normally be ignored, and so it is possible instead to measure impedance by measuring capacitance or reactance. However, to carry out more accurate measurement, measurement is carried out separately for reactance due to capacitance, and leakage resistance connected in parallel with capacitance, and a voltage applied to a conductor is measured using reactance and leakage resistance.